6. Silicate ceramics
6.0 Introduction
The main types of silicate ceramics are based either on alumosilicates ( kaolin- or claybased ceramic such as porcelain, earthenware, stoneware and bricks; system K2O-Al2O3SiO2) or on magnesium silicates ( talc-based technical ceramics such as steatite, cordierite
and forsterite ceramics; system MgO-Al2O3-SiO2). Special groups are zircon- and mullitebased fine ceramics (for electrical insulators) as well as low-thermal expansion ceramics in
the system Li2O-Al2O3-SiO2 (for thermoshock-resistant tableware), which have an extremely
narrow sintering interval and are therefore preferentially produced by glass-ceramic
techniques (i.e. devitrification of glasses by nucleation and growth). Silicate ceramics are
conventionally subdivided into coarse or fine and, according to water absorption, into dense
(< 2 % for fine and < 6 % for coarse) or porous ceramics (> 2% and > 6 %, respectively).
6.1 Kaolin- and clay-based ceramics and dental porcelain
In the ternary raw material diagram (kaolin/clay–feldspar–quartz) the composition of all
kaolin- and clay-based ceramics lies in the mullite field, that of dental porcelain in the leucite
field.
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Main types of dense silicate fine ceramics:
o Hard porcelain (the majority of middle-European porcelains – common firing
temperatures approx. 1400  50 °C).
o Soft porcelain (e.g. old Asian and “vitreous china“, firing temperature 1200–
1300 °C, higher content of fluxes).
o Bone china (English, based on up to 50 % bone ash as a raw material – apart
from kaolin, quartz), frit porcelain (French, based on glass frits, 1150 °C) and
“parian“ (unglazed ornamental porcelain with a low kaolin content of < 40 %).
o Dental porcelain (feldspar content approx. 80 %, kaolin < 5 %, firing
temperature < 1250 °C).
o Electrotechnical porcelain for insulators; usually containing alumina (instead
of quartz in earlier types of electrotechnical porcelain) in order to increase the
mechanical and electrical strength (firing temperature approx. 1250 °C).
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Hard porcelain:
o Porcelain is a densely sintered (defined by a water absorption < 2 %), white
and translucent fine ceramic material prepared from natural raw materials;
typical raw material mixture for hard porcelain: 50 % kaolin (part of which can
be replaced by plastic clay), 25 % quartz and 25 % feldspar (preferentially Kfeldspar).
o Hard porcelain: after firing at 1400  50 °C (with a relatively broad sintering
interval) the final ceramic body consists of min. 50 % glass phase, max. 25 %
mullite and max. 25 % residual quartz (which can be partly transformed to
cristobalite); typical properties: density 2.3–2.5 g/cm3, tensile modulus 70–80
GPa, Poisson ratio 0.17, flexural strength up to 100 MPa, thermal conductivity
1.2–1.6 W/mK, thermal expansion coefficient 4–6106 K1.
o At temperatures < 1100 °C the clay minerals (mainly kaolinite) dehydrate (
metakaolinite above 500–600 °C) and form transient phases by releasing silica
( defective spinel phase above 900–1000 °C), quartz undergoes polymorphic
transitions and mixed K-Na-feldspars (perthites) may homogenize; at
temperatures > 1100 °C: formation of feldspar melt causing liquid phase
sintering (vitrification), partial dissolution of quartz ( viscosity increase) and
formation of mullite, either directly from the clay minerals (primary mullite) or
by reaction of the clay minerals with the feldspar melt (secondary mullite).
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Other types of kaolin- and clay-based silicate ceramics:
o Earthenware: porous, non-transparent fine ceramics with a white or colored
body; typically fired at 1200 ± 50 °C and glazed with a PbO-containing glaze
in a second firing cycle (at approx. 1100 °C); typical raw material
compositions are 50–55 % clay, 40 ± 5 % quartz and 5–10 % feldspar;
commonly used for tableware and tiles (for the latter, however, firing is a
single-step process at about 1100 °C); “faience“ is earthenware with a white
body, “majolika“ with a colored body, “terracotta“ is coarse earthenware.
o Stoneware: porous, non-transparent coarse ceramics with a colored body,
typically fired at 1250 ± 50 °C (for sanitary ware, floor tiles and sewer pipes
with a brown NaCl glaze); note, however, that another variant of stoneware
(for tableware, tiles and chemical vessels) is a dense fine ceramic (vitrified
stoneware); sanitary ware is between porcelain and stoneware.
o Bricks: porous coarse ceramics, produced from cheap, local clays and loams,
typically fired at 900–1000 °C; the loams should not contain pyrite and sulfates
 CaSO4 near the body surface ( hydratation  volume expansion), neither
calcite (CaCO3)  may remain unreacted as free CaO after firing (
hydratation  volume expansion); important properties: frost resistance
(requires low porosity) and thermal insulation (requires high porosity).
6.2 Talc-based technical ceramics
All talc-based technical ceramics (ternary phase diagram MgO-Al2O3-SiO2) require precise
firing control (narrow sintering interval of a few °C).
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Steatite ceramics: basic raw materials – talc and clay (+ feldspar or BaCO3); desired
phase – protoenstatite in approx. 30 % glassy matrix; firing temperature 1350–1370
°C; problem to be controlled: transformation of the high-temperature proto- into (lowtemperature) clinoenstatite, accompanied with volume expansion.
Cordierite ceramics: basic raw materials – talc, clay and Al2O3; desired phase –
cordierite in a glassy matrix; self-glazing effect by non-wetting melt exuded onto the
surface; low thermal expansion coefficient (2106 K1). Note that only silica glass and
Li2O-Al2O3-SiO2 (glass-) ceramics exhibit lower thermal expansion coefficients (<
0.5106 K1)  higher thermal shock resistance.
Forsterite ceramics: basic raw materials – talc and clay (+ MgCO3); firing is less
sensitive with respect to temperature (since at 1360 °C only a small amount of eutectic
melt is formed and this amount does not change very much with temperature), but
very sensitive to changes in composition; high thermal expansion coefficient (11106
K1) enables welding with metals ( vaccuum electrotechnics).
Complex exercise problem: Use the ternary phase diagrams of the systems K2O-Al2O3-SiO2
and MgO-Al2O3-SiO2 to discuss the phase composition of hard porcelain, cordierite ceramics,
forsterite ceramics and steatite ceramics. Additional explicit questions:
a.) What are the essential high-temperature reactions for transforming the raw material
mixtures into the final phase composition of these ceramic products after firing ?
b.) Which microstructural features contradict the prediction from the equilibrium phase
diagrams and what are the reasons for the occurrence of non-equilibrium phases ?